NAM is the official provider of online scientific reporting for the
8th International AIDS Society Conference on HIV Pathogenesis, Treatment
and Prevention (IAS 2015), which will take place in Vancouver, Canada,
19th-22nd July 2015.

The proof of concept for this approach to HIV control in humans comes from the case of the 'Berlin patient', who received a bone marrow transplant from a donor with natural resistance to HIV infection. The donor was homozygous for the CCR5 delta 32 mutation, meaning that cells potentially vulnerable to HIV infection would never display the CCR5 receptor necessary for HIV to gain entry to that cell. As a result of complete ablation of the recipient's own stem cells by chemotherapy, his CD4 cells were replaced by cells derived from the donor's CCR5-lacking population. Over three years after the procedure the recipient remains HIV-free without antiretroviral treatment, and has been described as "functionally cured" by physicians.

However, the chance of reproducing this outcome using transfer of donor cells is very low due to the shortage of potential donors who are both HLA-compatible (essential for avoidance of graft-versus-host disease) and CCR5-delta32 homozygous. In any case, the essential elements that contributed to this functional cure are still not fully understood.

The introduction of modified genes into stem cells harvested from a person's bone marrow will be a necessarily individualised treatment, but the costs of gene therapy are likely to come down in the future, and if experimental approaches prove successful in controlling HIV without antiretroviral drugs, gene therapies may deliver a cost-effective form of treatment in the future.

The first human studies of gene therapy have sought to modify the expression of the CCR5 receptor. Sangamo Biosciences is developing a zinc finger nuclease which prevents CCR5 expression; study results have been presented at a number of conferences including ICAAC 2011 and CROI 2013 showing that the procedure is safe and that it results in long-term gains in CD4 cell numbers in people also taking antiretroviral therapy.

Two papers presented at CROI 2013 described the use of modified immune cells that make their own peptides (short chains
of amino acids) and act as fusion inhibitors, similar to the drug T-20
(enfuvirtide, Fuzeon). In one
experiment these cells, which essentially made their own anti-HIV drug, acted
as a treatment or therapeutic vaccine, curbing the viral load of a laboratory
virus engineered to be far more virulent than human HIV; in the other, they
prevented infection by a similar virus.

In the first study, rhesus monkey stem cells, the progenitors of
T-cells and all immune cells, were genetically altered to express a fusion
inhibitor peptide called mC46. While not able to prevent infection in monkeys
challenged with a highly pathogenic monkey/human SHIV (a laboratory-manufactured
virus combining parts of both the human and simian immunodeficiency virus genomes, created for
research purposes), it did
produce infections characterised by viral loads 2.5 and 3.15 logs lower than in
the control group of monkeys.

In another study, human CD4 cells had a fusion-inhibitor peptide
called C34 attached to their co-receptors (CCR5 or CXCR4). These were able to
resist infection by HIV in the test tube.

In the first paper, researchers from the Fred Hutchinson
Cancer Research Institute in Seattle took haematopoeic stem cells (HSCs),
bone-marrow cells that are the progenitors of all blood cells, from two pigtail
macaque monkeys and transformed them genetically by splicing an inserted gene
sequence for mC46 into their CCR5 receptor gene.

They then injected the cells back into the monkeys. One monkey
had 20%of its HSCs replaced by the C34-producing cells and the second had over
50% replaced.

A week later, they infected them and two control monkeys
with a particularly lethal strain of genetically engineered human/monkey SHIV, which destroys CD4 cells fast and usually develops a steady-state viral
load in the order of several million copies/ml.

In the control animals, their CD4 counts declined from around
600 cells/mm3 before infection to between 10 and 50 cells/mm3
within two to three weeks. In the monkeys with mC46, the CD4 cell count dipped
to about 100 cells/mm3 within two weeks of infection, but then rose
slowly back to pre-infection levels over the next six months.

At the time of highest SHIV viral load and fewest CD4 cells,
90% of the CD4 cells in the mC46 monkeys had the fusion-inhibitor-generating
insert in them, which is what one would expect, given that SHIV so decimates
non-mutated CD4 cells.

What was unexpected to the scientists, though, was that after
the period of peak viral load, the non-mutated CD4 cells made a partial
recovery – in one monkey to about 60% of all CD4 cells and in the
other about 20%. This is promising, as it shows that one would not need to
replace all or even most of the CD4 cells in the body with HIV-resistant ones
in order to contain an HIV infection, and that increasing numbers of
non-resistant cells does not lead to a new burst of virus.

As we said, SHIV reproduces furiously and peak viral load in
all monkeys ten days after infection was one billion copies/ml. After that,
viral load in the control monkeys declined to about half a million in one and
about ten million in the other. In the mC46 monkeys, it fell to about 100,000 in
the monkey with 20% of its cells replaced by mC46 cells and down to a few
hundred in the one with more than 50% of its cells replaced.

Viral load was about 320-fold lower (2.5 logs) in the first
monkey and about 1400-fold (3.15 logs) lower in the second. This
would transform a typical HIV viral load in an untreated human of 70,000
copies/ml to 50 copies/ml before any ARVs were taken.

Although there was a partial recovery of unmutated CD4 cells in
the blood, memory cells in the lymph nodes that form the ‘reservoir’ of
proviral HIV DNA remained predominantly the HIV-resistant mC46 cells, which is exactly where one would want
them to be.

Fusion-inhibitor human cells resist viral infection in test tube

The second experiment was a test-tube one, using human CD4
cell cultures. Researchers from the University of Pennsylvania physically
attached another fusion inhibitor peptide called C34 to the CCR5 or CXCR4
co-receptors. They found that the cells could not be infected with HIV.

Interestingly, although HIV attaches preferentially to one or
other of these co-receptors, they found that the action of the fusion inhibitor
peptide was non-specific: whichever receptor they attached it to, whether in
cells with one or both co-receptors, the cells became resistant to infection,
so the technique could work with HIV of R5 or X4 type. Its action was specific
to the co-receptors, though: attaching it to the CD4 molecule produced no
effect.

The co-receptors on the cells could still do their biological
job of attaching to the immune-activation molecules (MIP-1 beta and SDF-1) that
they attach to in the body, which indicates less likelihood of toxicity.

The researchers now hope to produce genetically altered cells
that produce the fusion inhibitor peptide as part of their co-receptors, as in
the previous study, and to move into animal experiments.

NAM is partnering with gTt (Barcelona), GAT (Lisbon) and LILA (Como) to deliver the CROI 2013 bulletins, which have also been made possible thanks to support from Bristol-Myers Squibb. NAM’s wider conference news reporting services have been supported by Abbott, Boehringer Ingelheim, Janssen, Roche and ViiV Healthcare. The funders have no editorial control over the content of the materials.

NAM’s information is intended to support, rather than replace, consultation with a healthcare professional. Talk to your doctor or another member of your healthcare team for advice tailored to your situation.